ISO BYZANTINE CHURCHES CHAP. compartments, alternately pierced by windows. The intermediate compartments correspond to the piers, and the dome is therefore twelve-sided on the exterior with angle half columns and arches in two orders. Internally the dome arches are recessed back from the lower wall face and spring from a heavy string-course. They were originally pierced on the north, south, and west sides by three windows similar to those in the west dome arch of S. Andrew (p. 114). The west side is now occupied by the wooden balcony of a Turkish house built over the narthex, but there are no indications of any gallery in that position. Below the dome arches the central area communicates with the surrounding ambulatory on the north, west, and south sides by large semicircular arches corbelled slightly out from the piers. On the east side the dome arch is open from floor to vault, and leads by a short berna to a five-sided space covered by a dome and forming a kind of triangular apse, on the south-eastern side of which is the mihrab. As is clearly shown by the character of its dome windows and masonry, this structure is a Turkish addition taking the place of the original three eastern apses, and is a clever piece of planning to alter the orientation of the building. The ambulatory on the three sides of the central square is covered by barrel vaults on the sides and with cross-groined vaults at the angles. To the east it opened into the eastern lateral chapels, now swept away, though the passage from the prothesis to the central apse still remains. On the north side of the church is a passage in three bays covered by dome vaults on transverse arches, communicating at the west end with the inner narthex, and at the east terminating in a small chapel covered by an octagonal drum dome. The upper part of the apse of the chapel is still visible on the exterior, but the lower part has been destroyed and its place taken by a Turkish window. The floor of the eastern part of the church is raised a step above the general level, this step being carried diagonally across the floor in the centre part so as to line with the side of the apse containing the mihrab.